text: First attempts to synthesize 2DPs date back to the 1930s when Gee reported interfacial polymerizations at the air/water interface in which a monolayer of an unsaturated fatty acid derivative was laterally polymerized to give a 2D cross-linked material. Since then a number of important attempts were reported in terms of cross-linking polymerization of monomers confined to layered templates or various interfaces. These approaches provide easy accesses to sheet-like polymers. However, the sheets' internal network structures are intrinsically irregular and the term "repeat unit" is not applicable (See for example:). In organic chemistry, creation of 2D periodic network structures has been a dream for decades. Another noteworthy approach is "on-surface polymerization"  whereby 2DPs with lateral dimensions not exceeding some tens of nanometers were reported. Laminar crystals are readily available, each layer of which can ideally be regarded as latent 2DP. There have been a number of attempts to isolate the individual layers by exfoliation techniques (see for example:).